The Glaciogenic System of the Ordovician Reservoirs in Sbaa Basin (SW Algeria)

Lena Dauphin¹, Eric Portier¹, and Guy Desaubliaux^2
1Exploration Production Department, Gaz de France, Saint Denis La Plaine, France
²Geology-Geochemistry Department, IFP, Rueil Malmaison, France

Glaciogenic reservoirs represent 0.5% of worldwide hydrocarbon fields. In the Sbaa Basin (SW Algeria) where Gaz de France (operators) and Sonatrach are partners, the main gas bearing reservoirs are constituted by glacial Ordovician sandstones deposited during the deglaciation of a giant icecap covering the Gondwana continent (Hirnantian event <1Ma).

Despite numerous available data (coring, logging, 3D-seismic), classical sequence stratigraphy method was insufficient to build a realistic 3D geological model. Because of specificity of such environments (rapid autocyclic oscillations, very high sedimentary charges, abrupt changes and extreme facies variability), cores and outcrops analyses revealed fundamental to understand the reservoir architecture.

Historically, stratigraphy of the Ordovician glacial deposits in the Sbaa Basin corresponds to the usually named “Unit IV”. No subdivision has never been attempted. Sedimentary study allowed to assert that the glacial deposits in this area correspond to a glaciomarine proximal to distal system generated by very high discharge of sediments, at the outlet of a wide dynamic ice stream. Two main glacial cycles were identified, separated by an interglacial stage. Each glacial cycle is composed by striking sequences, recognized in various wells of the basin. These sequences can be divided into elementary sequences, which number depends on position, distance to ice-front, and preservation. Sequential organization, and facies associations, allowed to characterize the Ordovician glacial system at the field scale and the basin scale.

Recently, these 2 main glacial cycles have been recognized in the eastern part of Anti-Atlas (Morrocco). These new outcrops confirm the sequential organization and the facies model of Sbaa Basin. This division is coherent with the last proposals of authors for the Late Ordovician Gondwana platform*.

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